This invention relates to the control of a diesel engine, and more specifically, relates to the use of skip firing of the engine to reduce smoke emissions.
The technique of eliminating the firing of selected cylinders in an internal combustion or diesel engine is referred to as xe2x80x9cskip firingxe2x80x9d. Removing the fuel supply (and/or spark ignition in a spark ignition engine) from these cylinders prevents them from firing. This technique has been used in the prior art to improve certain aspects of engine performance. When skip firing is initiated, the fuel quantity removed from the skipped cylinders must be added to the firing cylinders so that the performance parameters of the engine remain unchanged.
Large self-propelled traction vehicles, such as locomotives, typically use a diesel engine to drive a three-phase alternator (having a rotor mechanically coupled to the output shaft of the engine) for supplying electric current to one or more traction motors having rotors drivingly coupled (through speed reducing gearing) to axle-wheel sets of the vehicle. When excitation current is supplied to the field winding of the alternator rotor, alternating voltages are generated in the three-phase stator windings. The three-phase voltages are applied to input terminals of at least one three-phase, bi-directional power rectifier. If the locomotive has DC traction motors, then the rectified voltage is supplied to the parallel connected armature windings of the traction motors via a link. If the locomotive is equipped with AC rather than DC motors, then an inverter is interposed between the power rectifier and the traction motors to supply variable frequency power to the AC motors.
For the purpose of varying and regulating the speed of the diesel engine, it is common practice to equip the engine with a speed regulating governor that adjusts the quantity of pressurized diesel fuel injected into each engine cylinder. In this way, the actual speed (RPM) of the crank shaft is controlled and corresponds to a desired engine speed which is associated with the desired engine horsepower. In a typical electronic fuel injection system, the output signal from the speed regulating governor drives individual fuel injection pumps for each cylinder, thus allowing the controller to individually control the fuel value (i.e., amount of diesel fuel) injected into each cylinder. The desired engine speed and load is set by manually operating a lever or handle on the throttle that can be selectively moved through eight motoring steps or notches by the locomotive operator. In addition to the eight power notches, the handle has an idle.
When not in use, the locomotive is typically parked with its engine running, its throttle in the idle position, and its main alternator developing no power (i.e., because there is zero traction load). The typical idle speed is high enough to power all engine-driven auxiliary equipment. Further, to conserve fuel, it is also a known practice to reduce engine speed below the regular idle setting (i.e., to a preselected low idle speed) such as 335 RPM (so long as the desired engine performance parameters remain within appropriate tolerance limits). Although the low idle speed conserves fuel and reduces overall stress on the engine, it also causes the engine to generate excessive smoke. Specifically, at the idle or low idle notch position, there is a low fuel value (i.e., amount of fuel) injected into the cylinder each time the cylinder is fired and, more importantly from the standpoint of smoke generation, a lower fuel pressure.
Fuel injection pressure is critical to smoke formation. Fuel injected at higher pressures breaks up or atomizes better as it enters the combustion chamber. Better atomization allows air to mix with the fuel creating a higher air-fuel ratio. The higher air-fuel ratio locally within the cylinder fosters complete burning and low smoke production. On a specific fuel injection system with a defined pump, nozzle, cam profile, and operating speed, the injection pressure is governed by the injection duration. As the injection duration is extended within the cam profile, the injection pressure goes up. Idle conditions have two disadvantages regarding fuel pressure. First, idle conditions are unloaded, so injection durations are very short. Also, idle engine speeds are generally low, which create lower cam velocities. Both conditions significantly reduce the injection pressure, causing an increase in smoke production.
Engine components (i.e., cams, bearings, pumps, injectors, etc.) are designed to a maximum peak injection pressure limit. This prevents making mechanical changes to the fuel system to increase idle injection pressure, such as, a faster cam profile or smaller injector spray nozzle holes. Such design changes would raise peak injection pressure at all operating points. This is not desirable because at full load (notch 8), the peak injection pressure would then exceed design limits.
The recent enactment of environmental statutes and the promulgation of related regulations by the Environmental Protection Agency require reduction in smoke emissions from diesel locomotives. Locomotive manufacturers are therefore directing attention to reducing smoke emissions to comply with these regulations.
The system and method of the present invention overcomes the limitations and disadvantages of the prior art with respect to the production of visible smoke during low power operations of diesel engines. By skip firing the diesel engine, the smoke emissions are reduced. But it is critical to determine the conditions under which skip firing can be implemented without adversely impacting the power required by the various locomotive systems. Even when the locomotive is parked at idle, certain auxiliary systems load the diesel engine and thus it is required that the engine operate at some minimal power output level. The present invention also provides an apparatus and method for overcoming engine speed transients caused by the initiation and termination of engine skip firing.